Re: [PATCH v3 1/2] fork: lock VMAs of the parent process when forking

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Jul 5, 2023 at 10:14 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 05.07.23 19:12, Suren Baghdasaryan wrote:
> > When forking a child process, parent write-protects an anonymous page
> > and COW-shares it with the child being forked using copy_present_pte().
> > Parent's TLB is flushed right before we drop the parent's mmap_lock in
> > dup_mmap(). If we get a write-fault before that TLB flush in the parent,
> > and we end up replacing that anonymous page in the parent process in
> > do_wp_page() (because, COW-shared with the child), this might lead to
> > some stale writable TLB entries targeting the wrong (old) page.
> > Similar issue happened in the past with userfaultfd (see flush_tlb_page()
> > call inside do_wp_page()).
> > Lock VMAs of the parent process when forking a child, which prevents
> > concurrent page faults during fork operation and avoids this issue.
> > This fix can potentially regress some fork-heavy workloads. Kernel build
> > time did not show noticeable regression on a 56-core machine while a
> > stress test mapping 10000 VMAs and forking 5000 times in a tight loop
> > shows ~5% regression. If such fork time regression is unacceptable,
> > disabling CONFIG_PER_VMA_LOCK should restore its performance. Further
> > optimizations are possible if this regression proves to be problematic.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Reported-by: Jiri Slaby <jirislaby@kernel.org>
> > Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/
> > Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
> > Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/
> > Reported-by: Jacob Young <jacobly.alt@gmail.com>
> > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624
> > Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >   kernel/fork.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index b85814e614a5..403bc2b72301 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
> >               retval = -EINTR;
> >               goto fail_uprobe_end;
> >       }
> > +#ifdef CONFIG_PER_VMA_LOCK
> > +     /* Disallow any page faults before calling flush_cache_dup_mm */
> > +     for_each_vma(old_vmi, mpnt)
> > +             vma_start_write(mpnt);
> > +     vma_iter_init(&old_vmi, oldmm, 0);
> > +#endif
> >       flush_cache_dup_mm(oldmm);
> >       uprobe_dup_mmap(oldmm, mm);
> >       /*
>
> The old version was most probably fine as well, but this certainly looks
> even safer.
>
> Acked-by: David Hildenbrand <david@redhat.com>

Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>